1. Field of the Invention
The present invention relates to charge detecting circuits, and more specifically to detecting-driving circuits for use in linear image sensors.
2. Description of related art
Conventionally, a typical linear image sensor includes a plurality of photodetectors such as photodiodes arrayed in a straight line. These photodetectors are connected to corresponding inputs of a multiplexer whose output is connected to an output amplifier. This amplifier ordinarily includes an integration capacitor and an on-off switch connected in parallel between the input and the output of the amplifier, so as to form an integration amplifier for converting an input current signal to an output voltage signal.
With the above linear image sensor, the multiplexer is operated to scan the photodetector array, so that the photodetectors are sequentially read out and outputted through the output amplifier. In this reading operation, after each photodetector is read out, the on-off switch of the integration amplifier is turned on to short-circuit the integration capacitor, so that the residual electric charge in each photodetector is cleared through the on-off switch.
In this conventional linear image sensor, however, scanning noises or feed-through noises are inevitably generated, which will greatly decrease the signal-to-noise ratio when the detected signal is very weak.
Further, a wiring capacitance is formed in a lead extending from the output of the multiplexer to the input of the amplifier, and in addition, since the multiplexer is composed of a plurality of analog switches such as transistor switches, the multiplexer itself has a parasitic capacitance. These wiring capacitance and the parasitic capacitance will hold a portion of the electric charge generated in each photodetector when the photodetector is connected to the output of the multiplexer to be read by the amplifier, and will continue to retain the above charge even after the photodetector is separated from the output of the multiplexer. As a result, the charge retained in the wiring and parasitic capacitances is mixed with a charge generated in another photodetector when the second photodetector is connected to the output of the multiplexer to be read by the amplifier. This mixing of charges will decrease the sharpness of a image reproduced on the basis of a photodetection signal detected by the image sensor.
The above mentioned residual charge retained in the wiring capacitance and the parasitic capacitance is discharged through the on-off switch connected between the input and the output of the integration amplifier when the on-off switch is turned on. However, since the amplifier has a substantial output impedance, the dischargeof the residual charge will need a relative long time of period. This means that for sharp reproduction image the on-off switch in parallel to the amplifier has to be maintained in on condition for a long time at each time one photodetector is read out. This results in decrease of the scanning speed for the multiplexer.
The above mentioned problems have been encountered not only in the linear image sensor but also in the other detecting circuits which include an array of detectors each responsive to a physical property.